To determine whether phenylbutazone or frusemide administration exacerbates the urinary excretory responses to high intensity exercise, 6 fully conditioned Thoroughbred mares were subjected to treadmill exercise at =120% of the V̇O2max after 4 treatments: 1) 4.4 mg/kg phenylbutazone i.v. 24 h prior to exercise (B); 2) 1.0 mg/kg frusemide i.v. 4 h prior to exercise (F); 3) 4.4 mg/kg phenylbutazone i.v. 24 h prior to exercise and 1.0 mg/kg frusemide i.v. 4 h prior to exercise (BF) and 4) placebo at both pre-exercise times (C). Urine was collected for 4.5 h before exercise, during exercise and for 2 h after exercise.
Urine flow (UF) and urinary sodium excretion (UNAex) increased and urine osmolality (Uosm) decreased following frusemide administration (F and BF only). UF decreased (B and C only) during exercise and subsequently increased from a pre-exercise value of 4.3 ± 0.4 (s.e.) ml/min (n=24) to 8.4 ± 4.1, 15.0 ± 3.6, 10.3 ± 4.2 and 9.5 ± 1.6 ml/min between 25 and 40 min following exercise for B, F, BF and C, respectively. This finding was only significant for F. Similarly, UNAex increased from a pre-exercise value of 0.05 ± 0.02 mEq/min (n=24) to 0.53 ± 0.34, 0.68 ± 0.30, 0.61 ± 0.38 and 0.67 ± 0.25 mEq/min between 25 and 40 min following exercise for B, F, BF and C, respectively. This finding was significant for B and F but not for BF or C. Uosm decreased following exercise in all treatment groups.
Considerable within and between treatment variation in the urinary excretory responses to high intensity exercise was observed and precluded finding differences between treatment groups. To investigate this variation further, values for UF and UNAex between 25 and 40 min post exercise (period of greatest post exercise diuresis and natriuresis) were compared with peak plasma lactate concentration. Positive linear correlations were found for UF (r=0.65) and for UNAex (r=0.65). Thus, the magnitude of the diuresis and natriuresis which follows high intensity exercise in horses is related, in part, to work effort, as measured by plasma lactate concentration.